The present invention involves an aerosol method for producing a palladium-containing particulate product, palladium-containing particulate products producible by the method, uses of the powders in the manufacture of electronic products, and electronic products so manufactured.
Palladium is widely used in the manufacture of electrically conductive thick films for microelectronic devices. A trend continues, however, to produce ever smaller particles for thick film applications. Generally, desirable features in such small particles include a small particle size; a narrow particle size distribution; a dense, spherical particle morphology; and a crystalline grain structure. Existing technology for manufacturing palladium-containing particles could, however, be improved with respect to attaining all, or substantially all, of these desired features for particles used in thick film applications. Improvements in the particles could result in significant performance advantages and/or cost savings when used to make electronic or other products.
One method that has been used to make small particles is to precipitate the particles from a liquid medium. Such liquid precipitation techniques are often difficult to control to produce particles with the desired characteristics. It is especially difficult by the liquid precipitation route to obtain particles having dense, spherical particle morphology and with good crystallinity.
Aerosol methods have also been used to make small palladium particles. One aerosol method for making small particles is spray pyrolysis, in which an aerosol spray is generated and then converted in a reactor to the desired particles. Spray pyrolysis systems involving palladium have, however, been mostly experimental, and unsuitable for commercial particle production. Furthermore, control of particle size distribution is a concern with spray pyrolysis. Also, spray pyrolysis systems are often inefficient in the use of carrier gasses that suspend and carry liquid droplets of the aerosol. This inefficiency is a major consideration for commercial applications of spray pyrolysis systems.
Additionally, palladium is easily oxidizable and has a tendency to oxidize during the manufacture of electronic devices. The oxidation of palladium during manufacture of electronic devices is problematic because volume expansion that accompanies oxidation can cause film cracking and delamination. It has been proposed that the susceptibility of palladium particles to oxidation is reduced by the addition of a small amount of an alkaline earth metal and by making the palladium particles as single-crystal particles to eliminate diffusion of oxygen along grain boundaries. Even with an alkaline earth additive, however, oxidation of palladium during the manufacture of microelectronic devices is still a significant problem and the cost of making single-crystal particles is high because of the high processing temperatures and long residence times required.
There is a need for improved palladium-containing powders, for improved methods of manufacturing palladium-containing powders and for improved products incorporating or made using improved palladium-containing powders.
The present invention provides powders of high quality, micro-size palladium-containing particles of a variety of compositions and particle morphologies, and with carefully controlled particle size and size distribution, and an aerosol method for producing the particles. The particles are useful for making a variety of products.
Through careful control of the aerosol generation, and in combination with other features of the present invention concerning manufacture of palladium-containing powders, the process of the present invention permits preparation of very high quality powders of, palladium-containing particles that preferably have a weight average size of typically about 0.1 micron to about 4 microns in size, and for many applications from about 0.2 micron to about 0.8 micron in size. The powders also have a narrow size distribution such that typically at least about 90 weight percent of the particles being smaller than about twice the weight average size of the particles. Furthermore, typically less than about 10 weight percent, and preferably less than about 5 weight percent, of the particles are smaller than about one-half the weight average size of the particles.
The invention includes both single-phase and multi-phase, or composite, particles useful for a variety of product applications, including for use as catalysts and in the preparation of thick film paste formulations, such as are used for depositing palladium-containing films during manufacture of various electronic and other products. Multi-phase materials may be in a variety of morphological forms, such as an intimate mixture of two or more phases or with one phase forming a surface coating over a core including another phase.
One preferred class of multi-phase particles includes a metallic palladium-containing phase and a nonmetallic phase, which frequently includes a ceramic material. The nonmetallic phase could be in the form of a coating around a core of the metallic phase, in the form of small domains dispersed in a matrix of the metallic phase, or in some other form. A variety of ceramic and other materials can be used to effect a variety of beneficial modifications to particle properties, such as a modification of densification/sintering properties for improved. compatibility and bonding with ceramic dielectric layers in electronic devices, and especially when the palladium-containing powder is used to make internal electrodes in multi-layer capacitors. An important use of multi-phase particles is to reduce film shrinkage during firing in the manufacture of electronic devices. One preferred group of ceramic materials for use in the multi-phase particles are titanates, as are frequently used in dielectric layers of multi-layer capacitors. Another preferred group of ceramic materials for use in multi-phase particles includes silica, alumina, titania and zirconia, and especially silica and alumina.
The palladium-containing powders may also be made with surprisingly high resistance to palladium oxidation, providing a significant advantage during the manufacture of thick film electronic products by reducing volume expansion during firing. This is particularly important in the manufacture of multi-layer capacitors, multi-chip modules and other cofired products where volume expansions due to palladium oxidation can result in significant film cracking and delaminations. The high oxidation resistance of the particles is particularly surprising because the resistance to palladium oxidation may be obtained without the use of alkaline earth metal additives. This is even more surprising because the particles may be made to exhibit good oxidation resistance even when the particles are polycrystalline. This is particularly advantageous because particles of high oxidation resistance may be made without the significant operating expense required to make single crystal particles. In that regard, the maximum average stream temperature in the pyrolysis furnace should typically be in a range of from about 900xc2x0 C. to about 1300xc2x0 C., although other ranges may be more preferred in some circumstances. Furthermore, although the powders exhibit high resistance of palladium to oxidation without any additives, it has also been found with the present invention that oxidation resistance may be further improved by the addition of small quantitites of tin to the particles.
The palladium in the powder is typically in a metallic phase, whether in a single phase or multi-phase particles. In one embodiment of the invention, the particles of the powder includes high quality palladium alloys, and especially alloys with silver. It has been found that the quality of the alloy is highly dependent upon processing conditions. When preparing particles including a palladium/silver alloy, the maximum average stream temperatures in a furnace reactor should be in the range of from about 900xc2x0 C. to about 1200xc2x0 C., with even narrower temperature ranges being more preferred for better control of alloy quality. A surprising result of the high quality alloy is that the palladium in the alloy shows a remarkable resistance to oxidation, which is particularly advantageous in many applications for the manufacture of electronic products.
The process of the present invention for making the palladium-containing particles involves processing of a high quality aerosol including a palladium-containing precursor. The aerosol includes droplets of controlled size suspended in and carried by a carrier gas. In a thermal reactor, typically a furnace reactor, the liquid of the droplets is vaporized, permitting formation of the desired particles in an aerosol state. According to one embodiment of the present invention, an aerosol at a high droplet loading and at a high volumetric flow rate is fed to a reactor, where particles are formed. In addition to the high droplet loading and high volumetric flow rate, the aerosol also includes a narrow size distribution of droplets such that the particles exiting the reactor also have a narrow size distribution, with preferably at least about 75 weight percent, and more preferably at least 90 weight percent, of the particles being smaller than about twice the weight average particle size.
With the process, and accompanying apparatus, of the present invention, it is possible to produce high quality palladium-containing powders at a high production rate using spray pyrolysis. This represents a significant advancement relative to the small laboratory-scale processes currently used.
An important aspect of the method of the present invention is aerosol generation, in which a high quality aerosol is produced having a controlled droplet size and narrow droplet size distribution, but at a high volumetric flow rate and with high droplet loading. An ultrasonic generator design is provided for generation of the high quality aerosol at a high generation rate.
The aerosol generation is particularly advantageous for producing high aerosol flow rates with droplets having a weight average size of from about 1 micron to about 5 microns, preferably with no greater than about 30 weight percent of the droplets being larger than about two times the average droplet size.
High quality aerosol production is accomplished also with high droplet loading in the aerosol. The droplet loading is preferably greater than about 5xc3x97106 droplets per cubic centimeter of the aerosol. Furthermore, the aerosol typically includes greater than about 0.083 milliliters of droplets in the aerosol per liter of carrier gas in the aerosol. This high droplet loading is also accomplished at a high aerosol production rate, which is typically at a rate of greater than about 25 milliliters of droplets of liquid feed per hour per ultrasonic transducer. Total aerosol flow rates are typically larger than about 0.5 liter per hour of liquid droplets at the high droplet loading and with the narrow droplet size distribution.
Aerosol generation for particle manufacture of the present invention is believed to represent a significant improvement for powder manufacture relative to current powder manufacture operations, which are mainly for experimental purposes. These laboratory-scale processes typically use aerosols at only low rates and normally without a high aerosol density. With the aerosol generator of the present invention, however, high rates of droplet production are possible with efficient use of carrier gas. In one embodiment, the aerosol generator includes a plurality of ultrasonic transducers underlying a reservoir of liquid feed that is ultrasonically energized during operation. The aerosol generator includes multiple gas delivery outlets, or ports, for delivery of carrier gas to different portions of a liquid feed reservoir, so that droplets generated from the different portions of the reservoir are efficiently swept away to form the aerosol. A preferred embodiment includes at least one gas delivery outlet per ultrasonic transducer.
The process and the apparatus of the present invention are also capable of producing palladium-containing powder at a high rate without high losses of palladium in the system. This is accomplished through careful control of process equipment and operating parameters in a manner to reduce system residence times to inhibit high production losses. An important aspect of the process of the present invention is operation of a pyrolysis furnace at high flow rates and in a manner to reduce the potential for losses of palladium. When operating at the high Reynolds numbers typically encountered in the furnace with the present invention, it is important to carefully control the furnace temperature and temperature profile. For example, it is important to operate the furnace so that both the average maximum stream temperature and the maximum furnace wall temperature are low enough to avoid an undesirable volatilization of components.
In a further aspect of the process and apparatus of the invention, the particles may be advantageously cooled for collection in a manner to reduce potential for palladium losses. The particle cooling may advantageously be accomplished with a very short residence time by radial feed of a quench gas into a cooling conduit through which the particle-containing aerosol stream flows. In this manner, a cool gas buffer is developed around the inner walls of the cooling conduit, thereby reducing thermophoretic losses of particles during cooling.
Also, the particle manufacturing process of the present invention is versatile and may be adapted for preparation of a variety of palladium-containing particulate materials for a variety of applications. In that regard, one embodiment of the present invention includes concentration of the aerosol by at least a factor of about two, and more preferably by a factor of greater than about five, before introduction of the aerosol into the reactor. In this manner, substantial savings may be obtained through lower heating requirements in the reactor, lower cooling requirements for product streams from the reactor and smaller process equipment requirements.
In another embodiment, the process of the present invention involves classification by size of the droplets in the aerosol prior to introduction into the pyrolysis furnace. Preferably, droplets larger than about three times the average droplet size are removed, and even more preferably droplets larger than about two times the average droplet size are removed.
In yet another embodiment of the present invention, the particles are modified following manufacture, while still dispersed in an aerosol stream, prior to particle collection. In one aspect, the particles may be subjected to a coating following manufacture, such as by gas-to-particle conversion processes. Preferred coating processes include chemical vapor deposition and physical vapor deposition. In a further aspect, the particle modification may involve a structural modification, such as a post manufacture anneal to improve crystallinity or to alter particle morphology.
The present invention also includes thick film paste formulations including the palladium-containing particles of the present invention and processes of manufacturing films from paste formulations. Also included in the present invention are methods for making electronic and other products using the palladium-containing particles and the products so manufactured. The present invention provides a variety of products made using powder of the present invention. These products include electronic devices, such as multi-layer capacitors and multi-chip modules, and other products, such as catalysts. Also, the powders are particularly useful for making high definition patterned circuit lines with a close line spacing.